Beams, trusses, joists, and columns are the typical structural members that support the weight or loads of structures, including buildings and bridges. Structural members may be manufactured from a variety of materials, including steel, concrete, and wood, according to the structure design, environment, and cost.
Wood structural members are now typically manufactured from multiple wood segments that are bonded together, such as in glue-laminated members, laminated veneer, parallel strand wood and I-beams. These manufactured wood structural members have replaced sawn lumber or timbers because the former have higher design limits resulting from better inspection and manufacturing controls. Wood is a desirable material for use in many structural members because of its various characteristics, including strength for a given weight, appearance, cyclic load response, and fire resistance.
Wood I-beams can be manufactured with a wide variety of structural components. Generally, wood I-beams include a pair of parallel wood or manufactured wood flanges that are secured along their lengths to a transverse web material that can be formed of wood, manufactured wood, or steel.
FIG. 1 is an exploded side elevation of a conventional steel web wood I-beam 10 of a type widely used as beams, floor joists, and roof joists. Steel web wood I-beam 10 includes an opposed pair of elongated wood flanges 12 and 14 that have positioned between them multiple straight steel tubes 16 arranged in a zig zag pattern as a substantially continuous tubular steel web 18. Wood flanges 12 and 14 typically are formed of multiple wood sections 20 that are joined at their ends by finger joints 22.
In this embodiment, the web members 16 are formed from steel tubes 16 having ends 24 that are flattened parallel to the plane of web 18 to fit within either slots 26 in top surface 28 of wood flange 12 or similar slots (not shown) in bottom surface 32 of wood flange 14. Pins 34 made of metal, wood, or plastic pass through the sides of wood flanges 12 and 14 and flattened ends 24 of the steel tubes 16 so that tubular steel web 18 is secured to flanges 12 and 14.
Conventional manufacturing practices provide steel web wood I-beams that are of generally high quality and have generally high design limits that include having adequate stiffness to meet application deflection criteria. These manufacturing practices typically specify the grade of wood used to form flanges 12 and 14, the type and size of steel tubes 16, the pitch of the zig zag pattern formed by steel web 18, the size and placement of pins 34 and 54, and the size and form of flanges 12 and 14.
In conventional manufacturing practices, wood flanges 12 and 14 are formed of high grade wood so that flanges 12 and 14 can withstand the tensile and compressive forces applied to the I-beam, the shearing forces exerted by pins 34 and 54, and the pressure in flanges 12 and 14 adjacent pins 34. Finger joints 18 have a strength that is typically at least about 60 percent of the strength of the wood that is joined together. High grade wood is also required so that finger joints 18 have at least a minimum strength to prevent a localized failure that could lead to total failure or collapse of the beam.
Knots and slope of grain variations are typically avoided in flanges 12 and 14 because they can initiate localized failure under compression. The limited availability of affordable high quality wood can greatly increase the cost of steel web wood I-beams or require use of oversized flanges of lower quality wood. Moreover, conventional steel web wood I-beams suffer from the disadvantage of failing completely whenever tension flange failure occurs.
Furthermore, stiffness is a major consideration in the design of wood I-beams. The flange construction and material of the reinforcement panel all contribute to beam stiffness.